Wireless backhaul networks are deployed to carry the traffic between a wireless access network and the core network. For example, as described in the above referenced related patent applications, a wireless backhaul network may comprise a plurality of Hubs, each connected to the wired core network, via Ethernet. Each Hub serves multiple remote backhaul modules (RBM), in a point to multipoint or point to point configuration, using a wireless channel. Each RBM is deployed close to an access network base station, such as a small cell base station, and connected to the base station via a cable. The Hubs are deployed at the locations where wired high capacity access to the core network is available, e.g. at a fiber point-of-presence.
In a wireless backhaul network, the term cluster refers to a number of RBMs and their respective serving Hub. Performance of an RBM, such as throughput, is contingent upon its received Carrier-to-Interference-plus-Noise Ratio (CINR) and the amount of bandwidth allocated to this RBM given a selected carrier. The received signal strength of an RBM is determined by the transmit power of a serving hub and the pathloss between the serving hub and the RBM. The received interference-plus-noise level of an RBM is determined by the transmit powers of all the interfering hubs and the pathlosses between interfering Hubs and the RBM. An RBM is affected by an interfering Hub when a desired signal and an interfering signal are transmitted over the same carrier frequency.
In frequency reuse of 1 multi-sector deployment, there are two main types of interference, namely intra-cell interference and inter-cell interference. The problem of joint scheduling has been extensively researched in multiple dimensions, e.g., time, frequency and space. Fractional frequency reuse techniques coupled with power management have been researched and many methods proposed in the literature to obtain a good performance tradeoff, the system performance of which, however, is far from an interference-free performance upper bound in terms of capacity and reliability.
To achieve the close-to upper bound performance in point-to-multi-point backhaul systems, a common approach is to use a larger spectrum (e.g., frequency reuse of 3). However, the spectrum is expensive and may not always be available for use. Another approach is to perform interference cancellation or rejection, which is generally computationally expensive and is not always effective especially with channel estimation errors. For example, techniques such as Interference Rejection Combining (IRC) or Maximal Radio Combining (MRC) may be used.
In typical wireless backhaul networks, Hubs and RBMs are deployed at fixed locations, and Hubs are located at elevated locations with sufficient height above obstacles or other environmental clutter. For example, in an urban area, Hubs may be positioned on a tall building or a rooftop, above the clutter. Each RBM is typically co-located with an access network base station, e.g. for a small cell base station, on a utility pole, sidewall of a building or other location below the roofline. Thus, typically there is not a direct Line Of Sight (LOS) between an RBM and a Hub.
According to system simulations with a typical wireless backhaul system setup, most of RBMs that are capacity challenged (0-3 b/s/Hz spectral efficiency) are in the sector-edge areas, which suffer from heavy intra-cell interference from a co-located Hub. Typically, the percentage of these capacity-challenged RBMs is in the range of 15 to 25%. If the interference created by this group could be cancelled, the overall system capacity would be close to upper bound performance for a system with a frequency reuse of 3.
Operation of dual carrier systems is described, for example, in an article by Gora, J.; Redana, S., entitled “In-band and out-band relaying configurations for dual-carrier LTE-advanced system”, 2011 IEEE 22nd International Symposium on Personal Indoor and Mobile Radio Communications (PIMRC), pp. 1820-1824, 11-14 Sep. 2011 and in an article by Gong, M. X.; Shiwen Mao; Midkiff, S. F., entitled “Load- and Interference-Aware Channel Assignment for Dual-Radio Mesh Backhauls”, Global Telecommunications Conference, 2008. IEEE GLOBECOM 2008. IEEE, pp. 1-6, Nov. 30, 2008-Dec. 4, 2008.
It is known to use dual carrier Multiple-Input Multiple-Output (MIMO) systems with a fixed antenna topology, such as WiFi 2.4 GHz/5 GHz.
Transmissions on different carrier frequencies usually exhibit different pathloss characteristics between the same pair of nodes. Different carrier frequencies generally have different spectrum usage characteristics, such as the amount of available bandwidth or transmit power masks, for example. To reduce or eliminate interference, a concept of joint scheduling can be utilized, to allow carrier hopping, in which a node is assigned to the carrier with a lower or zero interference level.
For dual-carrier fixed wireless backhaul networks, conventional dual carrier hardware, i.e. with a separate RF chain/separate RF front end and antenna system for each carrier, is typically not optimally used in all cell areas. There is a need for a practical scheme to more effectively use dual carrier hardware, and for systems and methods to reduce the number of link budget challenged and interference challenged RBMs in dual carrier backhaul networks.
Accordingly, an object of the present invention is to provide systems and methods for improved performance in dual-carrier wireless backhaul networks, particularly for wireless backhaul solutions comprising fixed or stationary nodes with directional antennas, including small-cell non-line-of-sight (NLOS) backhaul networks.